Separating device for mixtures of gaseous or vaporous substances, especially for separating a carrier gas from a fraction to be analyzed in a combined gaschromatograph and mass-spectrometer

ABSTRACT

This invention relates to a device for separating mixtures of gasses or vapors. The device utilizes a separating chamber having an inlet for admitting the mixture by viscous flow. The chamber has two outlets, one of which permits molecular flow and the other of which permits viscous flow. Separation occurs when the lower molecular weight fraction of the mixture predominantly leaves the chamber through the outlet permitting molecular flow while the higher molecular weight fraction leaves through the viscous flow outlet. The device may be employed, for example, to separate the helium carrier gas from a hydrocarbon where the mixture is eluted from a gas chromatograph and it is desired to concentrate the hydrocarbon for analysis by mass spectrometry.

United States Patent Brunnee et al.

[ 51 July 25, 1972 [54] SEPARATING DEVICE FOR MIXTURES OF GASEOUS ORVAPOROUS SUBSTANCES, ESPECIALLY FOR SEPARATING A CARRIER GAS FROM AFRACTION TO BE ANALYZED IN A COMBINED GASCHROMATOGRAPH ANDMASS-SPECTROMETER 221 Filed: Apr-H6, 1970 211 Appl.No.: 25,919

[30] Foreign Application Priority Data April 17, 1969 Germany ..P 19 19460.6

52 U.S.Cl ..ss/197,5s/17 51 men... ..B01d53/02 [58] Field of Search"210/65; 55/67, 197, 386, 464, 55/17 [56] References Cited UNITED STATESPATENTS 1,747,155 2/1930 Birdsall ..2l0/65 Primary Examiner-John AdeeAttorney-Wolf, Greenfield & Sacks [57] ABSTRACT This invention relatesto a device for separating mixtures of gasses or vapors. The deviceutilizes a separating chamber having an inlet for admitting the mixtureby viscous flow. The chamber has two outlets, one of which permitsmolecular flow and the other of which permits viscous flow. Separationoccurs when the lower molecular weight fraction of the mixturepredominantly leaves the chamber through the outlet permitting molecularflow while the higher molecular weight fraction leaves through theviscous flow outlet. The device may be employed, for example, toseparate the helium carrier gas from a hydrocarbon where the mixture iseluted from a gas chromatograph and it is desired to concentrate thehydrocarbon for analysis by mass spectrometry.

8 Claims, 8 Drawing Figures Patented July 25, 1972 3,678,656

2 Sheets-Sheet 1 FIG. I

I, p I I 3.3 FIG 5 2 2 b R FIG. 6

Patented July 25, 1972 3,678,656

2 Sheets-Sheet 2 FIG g rm, FIG 7 7O 6O 50 4O 3O 20 012 0'5 1' 2 5 16 2 0fi L, [ml/minJ SEPARATING DEVICE FOR MIXTURES F GASEOUS OR VAPOROUSSUBSTANCES, ESPECIALLY FOR SEPARATING A CARRIER GAS FROM A FRACTION TOBE ANALYZED IN A COMBINED GASCHROMATOGRAPH AND MASS-SPECTROMETER In theseparating device the first fraction e.g. the gas carrier gas helium ispredorninently exhausted through the first exit whereas the secondfraction eg a hydrocarbon together with a rest of the first fractionpredominantly flow through the second exit into the mass-spectrometer.

Inasmuch as the rate of output flow of the gas chromatograph isessentially higher than the admissible rate of inlet flow into themassspectrometer, one purpose of the separating device is to divert asmall portion of the output flow of the gaschromatograph for analysis bythe mass-spectrometer. By selective branching of the second fraction theconcentration of said fraction to be analyzed is increased whereas ahigh loss of the second fraction results when a non-selective partitionof the output flow from the gaschromatograph is utilized. By using aselective separating device it is possible to increase the sensitivenessof measurement and proof of evidence of fractions to be analyzed eg in agaschromatograph-mass-spectrometer combination.

The selective suction of the first fraction through the first exit of aseparating device can be performed in different manners.

In a separating device of the kind disclosed in German Pat. No.1,052,955, the selective suction of the first fraction is achieved bypassing the gas mixture to be separated through a nozzle-like openingand dividing the expanding stream by using a diaphragm which lets passthe core area and keeps back the outer jacket-like area of the stream.The core area is led into the mass-spectrometer whereas the jacket areais exhausted by a pump from the space between the nozzle and diaphragm,said space forming a separating chamber. The molecules of the firstfraction such as helium on account of its lower molecules weightdiffuses predorninently into the jacket area so that the first fractionis thereby predorninently exhausted by the pump.

In a separating device of the type described in British Pat. No. l,O65,l31, the predominant separation of the carrier gas helium as a firstfraction is attained by utilizing the pores of a porous wall portion ofthe separating chamber as the first exit. Where the width of the poresis equal to or less than the medium free way of the gas molecules, theflow-conductivity of the porous wall is equal to the reciprocal value ofthe square root of the molecular weights of the concerned gascomponents; thus the lighter weight carrier gas helium is exhaustedpredominently through the first exit.

The principal disadvantage of all these known separating devices is thatthe optimal output of the second fraction (to be analyzed) which can beobtained from the second exit is reached only in connection with acertain volume of gas inlet flow into the separating chamber. Proceedingon the assumption that the separation is carried out by a single-stageseparating device containing e.g. a porous wall as first output anddimensioned so that the inlet flow is viscous, with an inlet flow ofml/min of argon and helium and a second output flow leaded into themass-spectrometer the percental effective output e.g. the proportion ofthe quantity of Argon within the second output flow to the quantity ofArgon within the inlet flow is 26 but is only 3 with an inlet flow of 1ml/min.

In order to overcome this limitation of the measurability differentmethods have been applied:

1. A first method to overcome this limitation is to connect the exit ofthe gaschromatograph to enable by-passing the separating deviceimmediately to the inlet of the massspectrometer or'to insertinterchangeable throttles into the inlet and/or second exit of theseparating device. These methods are subjected to an undesirable loss ofmeasuring time.

. A second method is the insertion of an adjustable throttle into theline between the separating device and the massspectrometer. It ispossible thereby to enlarge the range of measurability, however, such athrottle generally prevents the inlet flow into the mass-spectrometerfrom being viscose. Thus the additional enrichment of the fraction to beanalyzed which would be obtained with a viscous inlet flow is lostbecause the exhaust from the mass-spectrometer runs off molecularly.

3. An enlargement of the range of measurability can also be attained byinsertion of an adjustable throttle valve into the pipe between theporous wall at the first exit and the pump for the first output,however, there is only a very small range of adjustability and moreoverthe disadvantage occurs that memory-effects disturb the measurement onaccount of a backwards directed difiusion from the space between thethrottle valve and the first exit.

The main object of the present invention is to provide a separatingdevice which has a simple construction and which enables an extensiveaccommodation to the different gas quantities thus enabling an optimaleffective output of fractions to be analyzed. a

In accordance with the invention the first exit is made adjustable topermit regulation of the quantity of the first output. Thus, asexperiments have shown, it is possible to vary the first output to ahigh degree and notwithstanding to maintain the molecular flowconditions which are necessary to obtain a separating efiect.

An embodiment of good adjustability is attained by using an elongatedslit as the first exit. In this case the amount of first output can beadjusted by changing the width of the slit. The device is constructed insuch a manner that the slit is formed by an edge and a wall arrangedparallel to the edge.

It is useful to form the separating chamber like a long channel one endof which is connected to the inlet and the other end of which isconnected to the second exit for viscous flow to the mass-spectrometer.

A simple and compact construction is attained by forming the separatingchamber like a ringshaped chamber with inlet and exit at diametricallyopposed points. In this case slit-exits as second exit are arranged atopposite sides along the ringshaped chamber.

A large length of the separating chamber combined with a compactconstruction may be obtained by connecting several separating chambersin series. A further improvement in separating effect may be secured bymaking the connections from separating chamber to separating chamber forviscous flow and by simultaneously making the velocity of flow withinthe connections so fast that this velocity of flow is faster than thevelocity of diffusion of the effective fraction.

Such a multistage arrangement can be additionally improved by making thefirst exits of the several separating chambers adjustable independentlyfrom one another so that an optimal adjustment may be reached in eachstage.

The separating channel may be formed like a spiral or helix. Then thefirst exit can also form a spiral or helix so that separating chamberand exit channel form a double-spiral or helix.

Ifthe separating chamber over its whole length or here and there isdimensioned for viscose flow then an elongation of the chamber resultsin an enlargement of the separating effect provided that the velocity offlow is higher than the velocity of difiusion of the effective fractionof the output at the first exit.

In the drawing are shown some embodiments of the invention.

FIG. 1 shows a first embodiment of agaschromatographmass-spectrometer-system with a separating deviceaccording to the invention,

FIG. 2 a cross-sectional view of the first exit of the separating deviceaccording to FIG. 1 on an enlarged scale,

FIG. 3 a cross-sectional view of a multistage separating device with acommon adjustment of the several stages,

FIG. 4 a cross-sectional view of a multistage separating device withseparated adjustment of the several stages,

FIG. 5 a separating device with a spiral separating chamber,

FIG. 6 a fractional cross-sectional view along line 6-6 of FIG. 5,

FIG. 7 a different embodiment of a separating device, and

FIG. 8 a diagram for illustration of the operation.

The invention now shall be illustrated in its application for analysisof gas by a combined gaschromatograph-mass-spectrometer-system. The gasmixture M eluted from the column of the gaschromatograph GC consists ofa first fraction e. g. the carrier gas helium and a second gas fractione.g. a hydrocarbon. The mixture enters an inlet E into a separatingchamber K of a separating device T. This separating chamber K isprovided with two exits, namely a fust exit A, also called waste exit orexhaust exit, and a second exit N. The second exit N is a throttle pipewhich is dimensioned so as to form a pipe for viscous flow, providednormal pressures. The second exit N is connected to the vacuum chamber Vof a mass-spectrometer MS which is held under high vacuum pressure by apump P,. The first exit A is connected to an exhaust channel R and anexhaust pipe S connects the exhaust channel R to a pump P Therefore abranching of the inlet flow M, takes place in the separating chamber K.A first exit flow or first output M is drawn off by pump P or is led toa regenerator, whereas the second exit flow or second output M issubjected to an analysis by measuring the mass spectrum of the gasmixture of exit flow M The inlet flow M, consists of the second fractionto be analyzed and the carrier gas helium as first fraction needed forthe gaschromatography.

In the separating device according to the invention the measurement overa wide range of inlet quantities is controllable because the first exitA which is dimensioned for molecular flow is made adjustable. For thispurpose the exit A is given the form of a long slit the width w of whichis adjustable. Thereby a large range of control can be attained with amost small first output if the slit of the first exit has been closedand with a big first output at the widest adjustment of the slit. Overthe whole range of adjustment the condition must be fulfilled that theflow through the first exit is molecular because molecular flow is thebasis for the supposition that the gas of less weight e.g. the carriergas helium will predominently pass the first exit. This condition isfulfilled if the width w of the slit is given the same order as or ismade smaller than the mean free path length of the molecules of thecarrier gas at the pressure existing within the separating device. Ifthedepth t of the slit is made sufficient small and the width w of the slitsmall enough to ensure molecular flow through the slit then it ispossible to obtain rates of first output within the required order (upto 100 ml/min).

With a device constructed according to the invention it was possible tochange the first output M by adjusting the slit width w between 0.02 and0.002 mm in such a manner that with an inlet flow M, between 60 ml/minand 2 ml/min the second output M was maintained at l ml/rnin. The deptht of the slit was approximately 0.1 mm and the length of the slit about100 mm. The pressure was at about 20 to 50 torr.

Having given a general description of a device according to the presentinvention the embodiments shown in FIGS. 1 to 7 now shall be describedmore in detail.

In all embodiments the separating device is provided with a casing 1consisting of several parts and having one or more separating chamberswith inlet E, first exit A, second exit N, an exit channel R connectedto the first exit A and an arrangement for adjusting the first exit A.

The inlet E is formed as a capillary one end of which is connected tothe column (not shown in the drawing) of the gaschromatograph GC whereasthe other end is connected to the separating chamber K (single-stagedevice FIG. 1) or to the separating chamber K, (FIG. 3) or K (FIG. 7) ofthe first stage in multistage devices. The second exit N also consistsof a capillary one end of which is connected to the separating chamber K(FIG. I) or to the separating chamber K (FIG. 3) or K (FIG. 4) of thelast stage in multistage devices whereas the other end is connected tothe vacuum chamber V of the mass-spectrometer MS. The slitshaped firstexits A of all embodiments with exception of FIG. 7 are formed by thecombination of an edge 2 and a wall arranged parallel with said edgewhereas in FIG. 7 this first exit is formed by two parallel edges. Thedepth t of the slit between edge 2 and wall 3 is made small in order toattain a high rate of flow even with a small width w of the slit.Preferably the depth 1 is made between 1 mm. and 0.001 mm.

The width w of the slit is made adjustable. The upper limit of width wis reached at the width where the flow through the slit changes frommolecular to viscous flow. With increasing width w the character of flowgradually changes into an area in which the separation device generallyfulfills the function of a flow divider without a selective separatingfunction. The lower limit of the width w is subject to the conditionthat the slit must not be completely shut. The remaining opening orwidth is defined by the surface conditions. A variation of the width wat the rate of to I can be reached already with simple means for surfacefinish.

Curve 0 of FIG. 8 shows the function B =flM,) where B is the effectiveoutput and M, is the inlet flow e.g. the proportion of quantities of theeffective fraction at the inlet and second exit whereby the width w ofthe first exit A is controlled in such a manner that the output at thesecond exit is constant independent on changes of the inlet flow M,. Thegain or relative efi'ective output from the second exit of the secondfraction to be analyzed highly increases with decreasing inlet flow M,and simultaneous throttling of first output M at the first exit. It istrue that the effective output will decrease if after having reached theminimum of width w the inlet flow M, continues to decrease because theviscous flow of the output M into the vacuum chamber of the massspectrometer will decrease more than the output M However, this decreaseof the efiective output starts at M, 2 ml/min from a very high value ofefiective output B 83 and therefore is of no weight as may be seen froma comparison with curve 11 of FIG. 8 illustrating the function B f(M,)of an analogous one-stage separating device of the kind having a porouswall as the first exit. In such a device the decrease of relativeeffective output B begins at the maximum of admissible inlet flow M, of10 ml/min so that at M, 1 ml/min the relative efiective output is only 3compared with approximately 60 in the separating device of the presentinvention in which besides the inlet flow M, into the separating devicemay be increased up to 60 ml/min.

The device according to the invention utilizes an adjustable first exitwhich makes it possible to adapt the total output at the first exitoptimally to the inlet flow M, thereby avoiding unnecessary lossesthrough the first exit and securing basically higher rates of relativeeffective output B than is possible with hitherto known separatingdevices having an unchangeable width w at the first exit.

In the first embodiment according to FIG. 1 and 2 the ringshaped edge 2is arranged parallel to wall 3 formed by the bottom side of anadjustable element 4 in the form of an smooth circular plate. Thisadjustable element may consist of metal, glass, ceramics or plastics; itis preferably made of quartz.

The width w of the slit between edge 2 and wall 3 is adjusted by anadjusting arrangement consisting of a resetting spring 5, an adjustingspring 6, pressure elements 7, 8, 9, a capshaped adjusting screw 10 andan resilient diaphragm 11 which forms a vacuum-tight closure of casing 1and transmits the movement of pressure element 7 to the adjustableelement 4.

FIGS. 3 and 4 show different embodiments of two-stage separatingdevices. Two separating chambers K,,K (FIG. 3) and K K (FIG. 4) withincasing 1.1 (FIG. 3) and 1.2 (FIG. 4) respectively are connected inseries. The first separating chamber K, (FIG. 3) and K (FIG. 4) are eachconnected to the inlet E and the second chamber K (FIG. 3) and K, (FIG.4) are each connected to the second exit N. Chambers K (FIG. 3) and K(FIG. 4) are connected by a channel 12.1 (FIG. 3) and 12.2 (FIG. 4)respectively. This connecting channel 12.1 (FIG. 3), 12.2 (FIG. 4) isdimensioned in such a manner that at the given pressures the flow fromthe first to the second separating chamber is viscose and the velocityof flow within the connecting channel is high compared with the velocityof diffusion of the second fraction of the first output. Thereby thetotal separating effect which is obtained is equal to the product of theseparating effects of the two stages. The first exits are connected to acommon exhaust channel R or R which are respectively provided exhaustpipes S and S The embodiments of FIG. 3 and 4 differ from the embodimentof FIG. 1 essentially in that the separating chambers K and K, (FIG. 3)and K and K (FIG. 4) are formed as long channels having one endconnected to the inlet whereas the other end is connected to the secondexit, and the first exit extends between inlet and second exit. Theseparating chambers are ringshaped channels in which inlet and exit areat diametrically opposed points. Another difference is that eachseparating chamber is provided with two second exits which are arrangedat opposite sides along the ringshaped chamber.

In the embodiment of FIG. 3 the edges 2 are supported by a conicaladjustable element 4.1 whereas the wall 3.1 opposite the edges 2 isformed by the inside of easing 1.1 as a conical surface parallel to theedges 2. The adjusting arrangement is provided with a resetting spring5.1, an adjusting spring 6.1 and an adjusting screw 10.1.

In the embodiment of FIG. 4 the second exits A of both separatingchambers K and K between the edges 2 and opposing wall 3.2 areseparately adjustable, so that a better adaption to different pressureswithin the two separating chambers may be attained. For this purpose theedges of the second exits of the separating chamber K of the first stageare arranged at the bottom side of a circular plateshaped firstadjustable element 4.2 whereas the edges of the second exits of theseparating chamber K, of the second stage are arranged at the bottomside of a ringshaped second adjustable element 4.3 which concentricallysurrounds the first adjustable element. Both adjustable elements areengaged by not shown resetting springs and separated adjusting springs6.2 engaging the adjustable element 4.2 is engaged by a centraladjusting screw 10.2 whereas the adjusting spring 6.3 engaging thesecond adjustable element 4.3 is engaged by a capshaped adjusting screw10.3 through a vacuum tight diaphragm l 1.2. The second exits A areconnected to a common channel system R which is connected by an exhaustpipe S to pump P In both embodiments of FIG. 3 and 4 the vacuum tightclosing is attained by a diaphragm 11.1.

In the embodiment of FIG. 5 and 6 the separating chamber K is formedlike a spiral. The separating chamber K and the spiralshaped exhaustchannel R connected to exhaust pipe 8;, are like a double spiral and areformed by a double spiral edge 20, 2b and the wall 3.3 of a platelikeadjustable element 4.4 which is adjustable relative to the edges 2a, 2bto adjust the width w of the second exit A.

The separating chambers in the form of long channels, especially thespiralshaped separating chamber K in FIG. 5 and 6 are preferablydimensioned for viscous flow from the inlet to the first exit in orderto secure an improvement of the separating effect. I

FIG. 7 depicts an embodiment in which the separating chamber K exists ofthe mouth areas in front of two coaxial tubes which are directed againstone another. The mouths are made as edges 2c, 2d and form a ringshapedexit. The width w of this slit which is shown unnaturally large in FIG.7 is adjustable by axial shifting of exit N which is shiftable within aneck 13 of easing 1.3. The exit N can be adjusted by an adjusting screw10.4 and is vacuum-tight connected to the casing 1.3 by a flange l4 andbellow 15. The casing 1.3 surrounds the exhaust chamber R. with which isconnected the exhaust pipe 8.. Modifications are possible within thescope of the invention; the embodiment with long channelshapedseparating chamber may be useful also in connection with separatingdevices which have a slit as second exit of fixed width w. The rate ofsecond output could be also adjusted by changing the length of the slit.

We claim:

1. A device for separating mixtures of gaseous or vaporous substancesinto a first fraction of lower molecular weight and a second fraction ofhigher molecular weight, the device comprising means forrning aseparating chamber, an inlet through which the mixture viscously flowsinto the separating chamber, the separating chamber having a movablewall spaced from a fixed wall to provide an elongated narrow slittherebetween, means coacting with the movable wall for changing the gapbetween the movable wall and the fixed wall whereby the narrow slitprovides a first exit whose gap is adjustable within the rangepermitting molecular flow of the first fraction through the slit to anexhaust passage, and the separating chamber having a second exitpermitting the second fraction to viscously flow out of the chamber.

2. the separating device according to claim 1 wherein one of the wallsproviding the first exit slit has a thin edge defining one side of theslit and the other wall has a relatively broad surface parallel to thethin edge and defining the other side of the slit.

3. The separating device according to claim 1, wherein the separatingchamber is in the form of a long channel having the inlet at one end andthe second exit at the opposite end.

4. The separating device according to claim 3, wherein the first exitextends substantially along the entire length of the channel.

5. The separating device according to claim 4, wherein the long channelis in the form of a spiral.

6. The separating device according to claim 5, wherein the devicefurther includes an exhaust passage forming a spiral contiguous to theseparating chamber and forming therewith a double spiral.

7. The separating device according to claim 2 wherein the separatingchamber is annular, and the inlet and second exit are disposed atdiametrically opposite locations on the annulus to obtain the greatestseparation therebetween.

8. The separating device according to claim 1, further comprising meansforming a second separating chamber, the second chamber having anindependently moveable wall spaced from a fixed wall to provide anelongated narrow slit therebetween whose gap can be adjusted, the twoseparating chambers being connected by a channel dimensioned for viscousflow whereby the velocity of flow in the connecting channel is greaterthan the velocity of diffusion of the second fraction across the firstexit.

1. A device for separating mixtures of gaseous or vaporous substancesinto a first fraction of lower molecular weight and a second fraction ofhigher molecular weight, the device comprising means forming aseparating chamber, an inlet through which the mixture viscously flowsinto the separating chamber, the separating chamber having a movablewall spaced from a fixed wall to provide an elongated narrow slittherebetween, means coacting with the movable wall for changing the gapbetween the movable wall and the fixed wall whereby the narrow slitprovides a first exit whose gap is adjustable within the rangepermitting molecular flow of the first fraction through the slit to anexhaust passage, and the separating chamber having a second exitpermitting the second fraction to viscously Flow out of the chamber. 2.the separating device according to claim 1 wherein one of the wallsproviding the first exit slit has a thin edge defining one side of theslit and the other wall has a relatively broad surface parallel to thethin edge and defining the other side of the slit.
 3. The separatingdevice according to claim 1, wherein the separating chamber is in theform of a long channel having the inlet at one end and the second exitat the opposite end.
 4. The separating device according to claim 3,wherein the first exit extends substantially along the entire length ofthe channel.
 5. The separating device according to claim 4, wherein thelong channel is in the form of a spiral.
 6. The separating deviceaccording to claim 5, wherein the device further includes an exhaustpassage forming a spiral contiguous to the separating chamber andforming therewith a double spiral.
 7. The separating device according toclaim 2 wherein the separating chamber is annular, and the inlet andsecond exit are disposed at diametrically opposite locations on theannulus to obtain the greatest separation therebetween.
 8. Theseparating device according to claim 1, further comprising means forminga second separating chamber, the second chamber having an independentlymoveable wall spaced from a fixed wall to provide an elongated narrowslit therebetween whose gap can be adjusted, the two separating chambersbeing connected by a channel dimensioned for viscous flow whereby thevelocity of flow in the connecting channel is greater than the velocityof diffusion of the second fraction across the first exit.